Piezoelectric vibrating beam transducers, frequently found in modern acceleration, force and pressure sensors, are driven into oscillation by internal strains created by voltages applied to electrodes located on the beams. In general, it is desirable for a beam to oscillate only in the single mode for which the beam and the associated transducer are designed. Oscillation in other modes dissipates energy into the support structure, and thereby decreases the oscillator efficiency. Additionally, this lost energy can affect the operation of other transducers mounted on the same structure (crosstalk), or excite resonances in the support structure that affect normal transducer operation.
The material of choice for most modern vibrating beam transducers is crystalline quartz. In this material, the electrodes create electric fields that are normal to the longitudinal axis of the quarts beam, and the electric fields cause forces directed along the longitudinal axis. These longitudinal forces are arranged such that they cause the beam to vibrate parallel to the electric field.
One technique for causing such a quartz beam to oscillate only in a predetermined mode is described in U.S. Pat. No. 4,469,979. The technique set forth therein is based upon a determination that when a quartz beam is vibrating, for example in its fundamental flexure mode, the strain in the beam is in one direction near the ends of the beam, is in the opposite direction near the center of the beam, and passes through zero (i.e., changes direction) at relative positions of approximately Y/L=0.22 and 0.78, where L is the beam length and Y is longitudinal position along the beam. The technique used in the above-mentioned patent is reverse the electrode polarity at the crossover points of 0.22 and 0.78, to thereby increase the likelihood that the beam will vibrate only in its fundamental flexure mode.